1. Field of the Invention
The present invention relates generally to methods for remediation of polluted bodies and sources of groundwater. More particularly, the present invention relates to a method and system for extraction of chemicals such as non-aqueous phase liquids from surfactant-based groundwater remediation effluent water.
2. Background Technology
Contamination of groundwater with liquid organic contaminants such as nonaqueous phase liquids (NAPL), including dense nonaqueous phase liquids (DNAPL) and/or light nonaqueous phase liquids (LNAPL), plagues sites in both the public and private sectors. For example, for many years little care was taken in the handling of organic solvents and other contaminants which were used in industry and at government installations, such as military bases. Through poor handling techniques and occasional intentional dumping, many industrial sites and military bases now have contaminated areas containing relatively high concentrations of contaminants such as chlorinated solvents or other organic liquids. If not removed, such contaminants can filter down into groundwater supplies, rendering the water unfit for consumption and other uses.
Such groundwater contamination is fairly common in groundwater sites, which are permeable bodies of rock that are capable of yielding quantities of groundwater to wells and springs. The Environmental Protection Agency (EPA), Region I, recently identified 20 groundwater sites requiring remediation, with an additional 10,000 contaminated groundwater sites believed to exist nationwide at industrial, municipal, and military installations.
Several methods have been tested at various scales to determine their effectiveness in removal of organic contaminants from groundwater, and several techniques, both in situ and ex situ, are currently employed for remediating contaminated groundwater. To date, most of these have been pump-and-treat methods where DNAPL liquids are involved. In such a method, wells are drilled into the contaminated area and contaminated groundwater is pumped above the surface, where it is treated to remove the contaminants. Such pump-and-treat methods typically suffer from low process efficiency due to the relatively low solubility of DNAPL in the groundwater and resulting long clean-up time.
Due to the general impractability of pump-and-treat methods, other technologies have been developed which utilize a process commonly referred to as enhanced solubilization. Such a process uses micellar surfactant solutions to increase the effective solubility of the dense non-aqueous contaminants to accelerate the rate of removal. The mechanism for solubilization displayed by surfactants arises from the formation of microemulsions by the surfactants, water, and the solubilized dense non-aqueous liquid molecules.
One such technology is surfactant enhanced groundwater remediation (SEAR), which is particularly effective in the removal of chlorinated contaminants such as trichloroethylene (TCE) and perchloroethylene (PCE) from contaminated groundwater. In the SEAR process, a surfactant and other process chemicals such as a thickener and alcohol are injected into a contaminated groundwater to enhance solubility of the organic contaminants. Following dissolution, the contaminants, surfactant, and other process chemicals are pumped back to the surface where the chemicals are stripped, and treated water is reinjected to the groundwater. The SEAR process is particularly suited to remediation of groundwater in groundwater sites having an aquiclude such as a clay layer, which prevents downward vertical migration of solubilized organic contaminants. While the SEAR process is effective in clean-up of source contamination, the process is not an endpoint for water treatment, but rather is primarily useful in the removal of concentrated contaminants.
The SEAR process generates effluent water containing high concentrations of PCE, TCE, surfactant, and other process chemicals. Typical TCE/PCE concentrations in the SEAR effluent range as high as 5000 ppm. Alcohol and surfactant concentrations may reach up to 12% and 4%, respectively. Effluent treatment is a significant cost burden to the SEAR process, with up to 40% of the total SEAR process cost arising from effluent treatment.
A related technique is surfactant enhanced groundwater remediation at neutral buoyancy (SEAR-NB), which is an enhanced pump-and-treat method for remediation of TCE and PCE contaminated groundwater that is disclosed in U.S. Pat. No. 5,993,660 to Shook et al. In this process, surfactant, alcohol, and other components injected into a contaminated groundwater, are optimized to provide a microemulsion with a substantially neutral buoyancy with respect to the groundwater. Thus, the SEAR-NB process is particularly suited to remediation of groundwater in groundwater sites that do not have an aquiclude to prevent downward vertical migration of solubilized contaminants.
Considerable chemical cost is associated with the SEAR-NB process. Significant capital and operating costs are associated with the operation of a SEAR-NB effluent treatment plant. Effluent treatment currently accounts for approximately 50% of the total cost of the SEAR-NB process, not including costs associated with the loss of high value chemicals. Such loss of high value chemicals has a significant detrimental impact on the overall economics of the SEAR-NB process.
Extraction of organic compounds from water using critical fluids and direct extraction of DNAPLs from solids have been previously demonstrated. For example, critical fluid extraction has proven an effective method for removal of organics from contaminated soils and regeneration of activated carbon. Techniques have also been developed for extraction of various hydrocarbons and other contaminants from aqueous solution. CF Technologies (Hyde Park, Mass.) has demonstrated steady state extraction of various organic compounds from wastewater with efficiencies in excess of 99% in a continuous extraction system. Extracted compounds have included acetone, methylene chloride, trichloroethylene, toluene, and methyl methacrylate.
Several technologies, including activated carbon adsorption, steam stripping, and membranes have been explored to determine their effectiveness in separation of DNAPLs and process chemicals from the SEAR or SEAR-NB effluent. These technologies are typically effective in removal of TCE and PCE from the effluent, but often result in destruction and loss of process chemicals.
Activated carbon, although effective in removal of organic contaminants from aqueous solution, is most effective in treatment of dilute, less than 1%, process streams. As the SEAR or SEAR-NB processes generate an effluent stream with relatively high total organic content, in excess of 16%, large volumes of granulated activated carbon (GAC) would be required to enact the separation. The large consumption of GAC would result in generation of a large volume of secondary waste requiring regeneration or incineration. GAC adsorption would ultimately result in SEAR or SEAR-NB process chemical destruction.
Although steam distillation or stripping can separate PCE, alcohol, and surfactant from aqueous solution, the surfactant is generally damaged by the high temperature operation and cannot be reused. Foaming of the surfactant during distillation can also occur, making the separation impossible. Steam distillation would also be costly due to large energy requirements to enact the desired separation.
In the SEAR or SEAR-NB processes, a micro-emulsion is formed in which surfactant molecules surround molecules of PCE in the contaminated groundwater in micellular fashion. These micelles are then solubilized into the aqueous phase and pumped out of the groundwater with the alcohol/surfactant/water flood. Membranes, although capable of enacting the separation of highly dilute DNAPL from water, are generally not capable of breaking micellular microemulsions such as those encountered in the SEAR or SEAR-NB processes. To enact a separation, the membrane must have access to the material to be removed. Therefore, before a membrane could be used to separate PCE from aqueous solution, the emulsion must first be broken. This typically requires further chemical addition or thermal destruction of the surfactant. Membrane treatment technologies are particularly sensitive to the presence of suspended solids and would require pre-filtering of effluent prior to treatment. In the presence of suspended solids, membrane flux rate is significantly reduced through plugging of the pores.
Thus, there is a need for improved methods of extracting process chemicals and contaminants from groundwater remediation effluent water.
The present invention provides a method and system for extracting non-aqueous phase liquids from groundwater remediation effluent water.
The present invention provides a method and system for extracting remediation process chemicals from groundwater remediation effluent water, which allows for reuse of the process chemicals in the remediation process.
The present invention also provides a critical fluid extraction method for separation and concentration of hazardous materials and high-value chemicals.
The present invention further provides a critical fluid extraction method which is a viable alternative for waste minimization and volume reduction in handling of SEAR or SEAR-NB process effluent.
The invention yet further provides a critical fluid extraction method which allows for process chemical recovery and recycle to reduce the environmental impact and cost of the SEAR or SEAR-NB process.
In accordance with the invention as embodied and broadly described herein, a method and apparatus are provided for extraction of chemicals from an groundwater remediation aqueous effluent. The extraction method utilizes a critical fluid for separation and recovery of chemicals employed in remediating groundwater contaminated with hazardous organic substances, and is particularly suited for separation and recovery of process chemicals used in surfactant-based remediation technologies such as the SEAR or SEAR-NB process. The extraction method separates and recovers high-value chemicals from the remediation effluent and minimizes the volume of generated hazardous waste. The recovered chemicals can be recycled to the remediation process or stored for later use.
In one aspect of the invention, a method for extraction of chemicals from an groundwater remediation aqueous effluent comprises contacting a critical fluid with an aqueous effluent including water and a plurality of chemical components in a first stage at a preselected first temperature and first pressure. At least one of the chemical components is thereby substantially dissolved in the critical fluid. The critical fluid having at least one of the dissolved chemical components is then separated from the aqueous effluent. After the separation, additional critical fluid is contacted with the remaining aqueous effluent in one or more additional stages, with at least one of the first temperature and first pressure being adjusted at each stage, so as to sequentially remove each of the remaining chemical components from the aqueous effluent at each stage.
In another aspect, one embodiment of an extraction apparatus is provided for implementing the method of the invention. The extraction apparatus is a series column system which is particularly suited for performing critical fluid extraction of DNAPL and process chemicals from SEAR or SEAR-NB process effluent. The extraction apparatus includes a three column or tower extraction cascade in which dissolved components are selectively and sequentially removed and recovered from the SEAR or SEAR-NB process effluent.
These and other features and advantages of the present invention will become more fully apparent from the following description and appended claims.